1. Field of the Invention
This invention relates to cellular radio systems.
2. Related Art
A typical cellular radio system includes a number of base stations linked together to form a network, the base stations being under the control of a mobile switching center which can also have a connection to a fixed network. Each base station has one or more antennas for providing radio coverage within an area around the base station. This area is known as a cell. Each base station is capable of radio communication with a number of mobile units operating in its cell.
In order to set up a call to a mobile unit, the mobile switching center sends out a paging signal to all the base stations. If the mobile unit which is being paged responds, the base station which receives the response allocates a channel, from a number of channels available to it, for communication with the mobile unit.
In a conventional cellular radio system a mobile unit does not hold on to the same resource (radio channel) throughout its call. As the mobile progresses from one cell to another within the network, handover from a resource in the old cell to another resource within the new cell is performed by switching to a new channel. This allows the mobile unit to roam over specific geographic areas without significant interruption to the communication path, while allowing re-use of released resources by other mobile units. Existing cellular radio systems are designed for use by individual mobile units making and receiving calls on a one-to-one basis, so that a mobile unit can be connected to one other mobile unit on the cellular network, or to one terminal on an interconnected fixed network (eg PSTN). When a call is set up, the mobile unit is assigned a channel (defined by e.g frequency and/or timeslot) which, for the duration of the assignment, only that mobile can use.
For some purposes, in particular in the emergency services (ESs) such as fire, police, ambulance, coastguard, mountain rescue etc., there is a requirement for a control center (the "dispatcher") to be able to call to all mobile units simultaneously (known as a "broadcast" service) or for one mobile unit to call all the others (an "all-informed" service). However, these services require access to these facilities for only a small proportion of their operational requirements. It is thus wasteful of resources to devote equipment and spectrum in the radio band exclusively to the provision of such services. Other services with field forces such as taxi and public transport operators, utilities such as gas and telecommunications companies, and dispatch companies, also have a need to communicate with several members of the field force at once.
Many of these services' other communication requirements can be met by existing cellular systems. For example, existing systems allow calls to be made between an individual mobile unit and the dispatcher, initiated by either party. However, existing cellular radio systems do not meet the requirements for the "broadcast" or "all-informed" service. It would be especially advantageous to support the requirements of the emergency services on a cellular system, because the emergency services' existing private networks have to have a high capacity to allow the system to cope with extreme situations, but this capacity is rarely required, and the system is generally under utilized.
Some cellular systems offer a supplementary service known as "multi-party calling" or "conference calling". This would allow a number of mobiles to communicate with the dispatcher and each other simultaneously. Call set-up can be initiated by any of the participants. However, multi-party calling presents some operational restrictions for the emergency services. In particular there is the need to set up calls individually; with the inherent time overhead, and the requirement for separate radio resources to be devoted to the system for each member of the multi-party call, which is wasteful of physical resources and can lead to capacity problems.
By the nature of their duties, the emergency services often have to deploy a large number of resources into a small area. This can put a heavy demand on the resources of the local base station of a cellular system if each unit requires its own channel. The base station might not be capable of meeting these capacity requirements, even if the emergency service mobiles are given priority over all other users. The nature of emergencies makes normal cell enhancement methods unsuitable for coping with these unpredictable sudden high demands.